(a) Field of the Invention
The present invention relates to an electrochemical cell. More particularly, the present invention relates to an electrochemical cell having excellent reversible capacity and cycle life characteristics.
This work was supported by the IT R&D program of MKE/IITA [Core lithium secondary battery anode materials for next generation mobile power module, 2008-F-019-01].
(b) Description of the Related Art
A battery as a power source for a portable electronic device and an electric vehicle has been increasingly required to have high performance and large capacity.
In general, a battery generates electric power by using electrochemical reaction materials at positive and negative electrodes. For example, a lithium secondary battery generates electric power due to chemical potential change, when lithium ions are intercalated and deintercalated from positive and negative electrodes.
The lithium secondary battery is fabricated by using a material capable of reversibly intercalating and deintercalating lithium ions as positive and negative active materials and charging an organic electrolyte solution or a polymer electrolyte solution between the positive and negative electrodes.
Recently, a nano-material has been actively researched as an active material for these batteries. Since the nano-material has a high surface area, it can provide higher electrode and electrolyte contact areas, which can lead to shorter diffusion paths with the particles and more facile intercalation for lithium ions.
A positive nano-active material has been reported to be prepared as a nanowire such as lithium- and Ni-deficient LixMn0.67Ni0.3O2 formed by chemical oxidation of pristine bulk LiMn0.5Ni0.5O2 powder. Since the positive active material is acid-treated, it has an extreme lack of lithium. In other words, the compound has x of less than 1. The nanowire has reversible capacity of about 160 mAh/g between 4.8 and 2 V at a current density of 20 mA/g.
Recently, among lithium composite metal oxides, Mn-rich lithium metal oxides such as in a Li[Li1/3-2x/3Mn2/3-x/3Mx]O2 positive electrode are currently receiving significant interest as a positive active material. These materials can provide initial capacity of greater than 200 mAh/g at 4.5V or higher. However, in spite of the capacity advantages, a rapid capacity fade incurs higher C rates. For example, capacity fade of over 50% is observed when the current is increased from 20 to 200 mAh/g in a Li[Ni0.2Li0.2Mn0.6]O2 active material. Furthermore, a Li[Ni0.41Li0.08Mn0.51]O2 nanoplate active material has been reported to be prepared in a co-precipitation method to improve high rate characteristics of a battery. However, there has still been research on improvement of high rate characteristics of a battery.